1. Field of the Invention
This invention relates to a two-way wireless messaging system for transmitting and receiving messages. More particularly, this invention relates to a wireless voice messaging system for transmitting and receiving encoded voice signals which are subject to variable data compression.
2. Background
Personal communication systems for enabling users to communicate with each other have become popular in the recent years. While in the past such systems have generally been realized using analog technology, it is desirable to instead use digital processing in order to use limited frequency resources more efficiently as well as reduce signal distortion and degradation and thus improve the overall quality of the voice signal.
In general, in a digital cellular telephone system for transmitting and receiving digital voice signals, an input voice signal is digitized and encoded to speech parameters. A variety of speech encoding/decoding methods have been known for processing digital voice signals. For example, MBE (Multi Band Excitation), SBE (Single Band Excitation), SBC (Sub-Band Coding), Harmonic Coding, LPC (Linear Predictive Coding), DCT (Discrete Cosine Transform), MDCT (Modified DCT) and FFT (Fast Fourier Transform) have been known as such a encoding/decoding method. In addition, CELP (Code Excited Linear Prediction), VSELP (Vector Sum Excited Linear Prediction), PSI-CELP (Pitch Synchronous Innovation - CELP) and RPE-LTP (Regular Pulse Excitation - Long Term Prediction) have also become known as a speech encoding method for digital cellular telephone system.
Such a digital cellular telephone system is called a circuit switched communication and two or more users can interactively communicate with each other in realtime. During the interactive communication, a signal channel is physically established exclusively for the communicating users. The cellular telephone system is relatively expensive because the users are charged for the exclusive use of the signal channel, for example, per minute basis. Even when a user just wants to talk for a short period to send a simple voice message, the service takes a high charge for using a cellular network in conjunction with PSTN (Public Switch Telephone Network).
Unlike a cellular telephone system for a realtime communication, packet switched communication systems are also known. Packet switched communication systems are considered as non-realtime communication systems. One of the non-realtime communication systems is a one-way pager which is capable of receiving a text-based short message. A two-way pager terminal is also known for providing limited transmitting capability as well as receiving capability of a text-based short message. For the non-realtime communication system, there is no need to establish an exclusive signal channel between the communicating users. Therefore, the charge for the system is based on the amount of data transmitted, for example, per byte basis. However, such a pager system cannot transmit a voice message.